Help with upload of website to wp

Hi
I have a website that I have installed wordpress on, and am able to activate themes plugins etc. However when I upload files for a wordpress theme on vouchers, I am unable to get the website to work. I know it is fairly straight forward but I cannot grasp this at all. I need any help on how to upload the website files please. I was ok with html before a long period in hospital, but not this wp which seems to be preferred. I can create databases/users etc. It's getting the files to work with wp I cant grasp. Thanks in advance. Mike